40 FLUIDITY. 



If the bodies are in the fluid form, they freeze upon being 

 cooled below the temperatures set against them. 



It may be added, in reference to this table, first, that in cer- 

 tain circumstances, liquids can be cooled down several degrees 

 below their usual freezing point before they begin to congeal. 

 Thus we may succeed, by taking certain precautions, in cooling 

 a small quantity of water, in a glass tube, so low as the tempe- 

 rature 8, or even as 5, without its freezing ; that is, 24 or 27 

 degrees under its proper freezing point 32. The water must 

 be cooled without the slightest agitation, and no sand or angular 

 body be in contact with it ; for the instant any solid body is 

 dropped into water cooled below its freezing point, or a tremor 

 is communicated to it, congelation commences, and the tempera- 

 ture of the liquid starts up to 32. But, on the other hand, 

 we cannot heat a solid the smallest fraction of a degree above 

 its proper melting point, without occasioning liquefaction. 

 Hence it is not the freezing of water, but the melting of ice, 

 which takes place with rigorous constancy at 32 Fahrenheit. 



All salts dissolved in water have the effect of lowering the 

 freezing temperature of that liquid. Common culinary salt 

 appears to depress this point lower than any other saline body ; 

 and the effect appears to be very closely proportional to the 

 quantity of salt in solution. A solution of 1 part of salt in 4 

 of water freezes at 4, and sea water, which contains l-30th of 

 its weight of salt, freezes at 28. 



But the principal fact to be adverted to in liquefaction, is the 

 disappearance of a large quantity of heat during the change. 

 Heat pours into a body during its melting, without raising its 

 temperature in the most minute degree. This heat, which 

 enters the body and becomes insensible or latent, merely 

 serves to melt the body. We are indebted to Dr. Black for this 

 observation, which involves consequences of greater importance 

 than any other announcement in the theory of heat. 



Before Dr. Black's views were made known, fluidity was uni- 

 versally considered as produced by a very small addition to the 

 quantity of heat which a body contains, when it is once heated 

 up to its melting point ; that a solid body, when it is changed 

 into a fluid, receives no greater addition to the heat within it 

 than is indicated and measured by the elevation of the mercury 

 in the thermometer. But Dr. Black objected to this opinion, 

 as inconsistent with many remarkable facts, when considered 



